1. Field of the Invention
The present invention relates to a sigma-delta (.tau..DELTA.) modulation circuit for an analog/digital (A/D) converter and a digital/analog (D/A) converter.
2. Description of the Related Art
An A/D converter, a D/A converter, and a signal transmission apparatus used in the technical fields of audio signal processing, data communications, etc. are constructed using, for example, a sigma-delta modulation circuit.
FIG. 7 is a view of the configuration of an ordinary sigma-delta modulation circuit.
As shown in FIG. 7, the sigma-delta modulation circuit comprises a subtractor 10, an accumulator 11, a binary comparator 12, a delay circuit 13, and a constant coefficient multiplier 14. The sigma-delta modulation circuit as a whole functions as a D/A converter for converting a digital input signal S.sub.IN to an output signal S.sub.OUT.
The subtractor 10, the accumulator 11, and the binary comparator 12 are digital type arithmetic and logic units.
The subtractor 10 subtracts a modulation signal S14 from the input signal S.sub.IN to generate a subtracted signal S10 and outputs it to the accumulator 11.
The accumulator 11 cumulatively adds the subtracted signal S10 from the subtractor 10 every sampling period to obtain a cumulative addition signal S11 and outputs it to the binary comparator 12.
The binary comparator 12 compares the cumulative addition signal S11 with a predetermined reference value. When the cumulative addition signal S11 is larger than the reference value, it outputs "+1" as the output signal S.sub.OUT, while outputs "-1" in other cases. This value itself is output as the output signal S.sub.OUT.
The delay circuit 13 outputs a delay signal S13 obtained by delaying the output signal S.sub.OUT, which is "+1" or "-1", by one sampling period and outputs it to the constant coefficient multiplier 14.
The constant coefficient multiplier 14 multiplies the delay signal S13 from the delay circuit 13 by a constant .DELTA.. As a result, when the delay signal S13 is "+1", it outputs "+.DELTA." as a modulation signal S14, while when the delay signal S13 is "-1", it outputs "-.DELTA.".
According to the sigma-delta modulation circuit shown in FIG. 7, the output signal S.sub.OUT is expressed as an approximation of the digital input signal S.sub.IN using the codes of "+1" or "-1". This signal can be regarded as an analog signal generated by adding a quantization error, which is small at the low frequency range and large at the high frequency range, to the digital input signal S.sub.IN. Therefore, by passing the output signal S.sub.OUT through an analog type low-pass filter, the circuit functions as a D/A modulation circuit as a whole.
Note that a case of a first order type sigma-delta modulation circuit is shown in FIG. 7, however, the basic operation principle is same in a sigma-delta modulation circuit of the secondary order or higher order in which only the portion of the subtractor 10 and the accumulator 11 in FIG. 7 is replaced by circuits having a higher order linear transfer function.
When using the sigma-delta modulation circuit shown in FIG. 7, for example in a communication apparatus, the amplitude of the output signal obtained in the sigma-delta modulation circuit is required to be controlled in some cases in order to adjust the amplitude of the modulation signal.
The sigma-delta modulation circuit does not comprise a gain adjustment function circuit in itself. A gain adjustment circuit (not shown) is provided outside the modulation circuit.
However, with this configuration, there is the disadvantage that the circuit becomes complicated and large in size. Particularly, in a sigma-delta modulation circuit used in a modulating portion of a communication device, the variable amplitude range is often narrow. Accordingly, while a large dynamic range is not required, there are strong demands for adjusting the amplitude by precise steps and simplifying the circuit configuration.
Similarly, when a sigma-delta modulation circuit is used as an A/D modulation circuit for the modulation of a communication apparatus, the amplitude range is often relatively narrow. Accordingly, while a large dynamic range is not required, there are strong demands for adjusting the amplitude by precise steps and simplifying the circuit configuration.